MPSC 2024 Prelims Paper-I · Question 55 of 100

Correct — option (1), all three statements are true. The stem asks the candidate to judge each printed statement as true or false — the words 'true' and 'false' are set in bold in the paper — and note that this question labels its statements with Roman numerals (I) to (III), and prints only three of them rather than the four the rest of the paper uses. Statement (I) is true: sound is a mechanical wave, a travelling disturbance of compressions and rarefactions that needs particles of matter to pass the disturbance along, so it cannot cross a vacuum. The classic demonstration is the bell jar, where an electric bell is set ringing inside a jar and the sound fades to nothing as the air is pumped out even though the hammer can still be seen striking. This is the property that distinguishes sound from light and radio waves, which are electromagnetic and travel through empty space. Statement (II) is true: frequency is the number of complete vibrations per second, and its SI unit is the hertz, named after Heinrich Hertz, one hertz being one cycle per second. Frequency is what the ear interprets as pitch, and the human audible range runs from about 20 hertz to about 20,000 hertz, with infrasound below and ultrasound above. Statement (III) is true: loudness is expressed in decibels, a unit that is one-tenth of a bel and is named after Alexander Graham Bell. The decibel scale is logarithmic rather than linear, so every increase of ten decibels represents a tenfold increase in sound intensity, which is why the enormous range the ear can handle compresses into a scale running from zero at the threshold of hearing to about 120 decibels at the threshold of pain. With no false statement among the three, the choice that affirms all of them is the answer.

• (2)Only (I), (II) is true — This accepts statements (I) and (II) but rejects statement (III), the use of the decibel for loudness — the one statement of the three that a candidate is most likely to doubt, because loudness is a sensation while the decibel measures a physical intensity level. The doubt is worth understanding but does not make the statement false: the decibel is the standard unit in which loudness is reported, it is the unit in which noise standards and hearing damage thresholds are written, and school and public usage are unambiguous on the point. Rejecting it also leaves the question with no false statement to justify the rejection.
• (3)Only (II) is true — This accepts only statement (II) and so rejects both the need for a medium and the decibel as the unit of loudness. The first rejection is the more serious, because the requirement of a material medium is the defining property of a mechanical wave and the reason space is silent; a candidate who doubts it may be confusing sound with the electromagnetic waves used to carry radio and television signals, which do travel through vacuum and which are what actually cross the space between a spacecraft and the earth. Both rejected statements are correct as printed.
• (4)(I), (II), (III) are false — This declares all three statements false, which would require sound to travel through a vacuum, frequency to be measured in something other than the hertz, and loudness to be expressed in some unit other than the decibel — three separate errors, each contradicting a settled definition. An option of this shape is worth a moment's thought only when the statements share a common false premise that would bring them all down together; here they are independent facts about three different aspects of sound, and each stands on its own.

Sound is a longitudinal mechanical wave: the particles of the medium oscillate back and forth along the direction in which the disturbance travels, producing regions of compression and rarefaction that move outward while the particles themselves stay near their original positions. Because the disturbance is handed from particle to particle, a material medium is indispensable, and the speed of sound rises as the medium becomes stiffer and its particles more closely packed — roughly 340 metres per second in air at ordinary temperature, about four times that in water, and faster still in steel. Three properties of the wave map onto three sensations. Frequency, measured in hertz, is perceived as pitch, and the audible band runs from about 20 to about 20,000 hertz. Amplitude determines the energy carried and is perceived as loudness, which is reported on the decibel scale; because the ear responds to an enormous range of intensities, that scale is logarithmic, with each ten-decibel step corresponding to a tenfold change in intensity. Waveform or harmonic content is perceived as quality or timbre, and is what distinguishes the same note played on two different instruments. The decibel scale also underpins environmental regulation, since ambient noise standards and permissible exposure limits are all written in decibels.

MPSC's general science questions on sound stay close to school-level material and test definitions rather than derivations, so this item is answerable from a settled block of facts about mechanical waves. What the Commission adds is a change of format: the true-or-false framing, the Roman numeral labels used on this question and the one before it, and a statement list of three items rather than four. Each of those is a small opportunity for a careless error — matching the wrong option to the wrong statement, or assuming a fourth statement exists. Reading the label system and counting the statements before turning to the options is the safeguard. Note too that where every printed statement is true, an all-inclusive option is a legitimate answer and should not be discounted merely because it looks too generous.

• Sound is a longitudinal mechanical wave and requires a material medium; it cannot travel through a vacuum, which is demonstrated by the fading of a bell ringing inside an evacuated jar.
• The SI unit of frequency is the hertz, named after Heinrich Hertz, defined as one cycle per second; frequency is perceived as pitch, and the human audible range is roughly 20 hertz to 20,000 hertz.
• Loudness is expressed in decibels, a unit equal to one-tenth of a bel and named after Alexander Graham Bell; the scale is logarithmic, so a rise of ten decibels means a tenfold rise in sound intensity.
• The decibel scale runs from about zero at the threshold of human hearing to about 120 at the threshold of pain, and prolonged exposure well below that upper figure can still damage hearing.
• The speed of sound depends on the medium, being about 340 metres per second in air at ordinary temperature and several times greater in water and in solids such as steel.

• Rejecting the decibel as the unit of loudness on the ground that loudness is subjective, when the decibel is the standard unit in which it is reported
• Assuming sound can travel through space because radio signals from spacecraft reach the earth, when those signals are electromagnetic rather than mechanical waves
• Discounting an option that affirms every statement, when a list can legitimately contain no false item
• Misreading which statement an option refers to when the labels are Roman numerals and only three statements are printed rather than four

Sound appears in MPSC general science as definitional recall — the unit of a quantity, the property a sensation corresponds to, the medium requirement — set either as a direct question or as a true-or-false statement list of this kind. The neighbouring topics that recur are ultrasound applications, echo and reverberation, and the speed of sound in different media. Because the factual content is small and stable, the marks in this area are lost to format rather than to ignorance: a missed negation, a misread label, or an all-inclusive option rejected out of suspicion.

No directly related past PYQ was found.

• practice — not a real PYQ

  An increase of ten decibels in the sound level corresponds to which of the following changes in sound intensity ?

  • (a)An increase of ten units
  • (b)A doubling of intensity
  • (c)A tenfold increase in intensity
  • (d)A hundredfold increase in intensity

  Answer(c) A tenfold increase in intensity — the decibel scale is logarithmic, so each ten-decibel step multiplies the intensity by ten and a twenty-decibel rise multiplies it by a hundred. This compression is what allows the whole range from the threshold of hearing to the threshold of pain to be expressed on a scale running from about zero to about 120.

• practice — not a real PYQ

  Which one of the following statements about the propagation of sound is correct ?

  • (a)Sound travels fastest in air and slowest in solids
  • (b)Sound travels faster in solids than in liquids, and faster in liquids than in gases
  • (c)Sound travels at the same speed in all media
  • (d)Sound can travel through a vacuum but not through a gas

  Answer(b) Sound travels faster in solids than in liquids, and faster in liquids than in gases — a stiffer medium with more closely packed particles passes the disturbance along more quickly, so the speed is about 340 metres per second in air, roughly 1,500 in water and several thousand in steel. Sound cannot travel through a vacuum at all, since it requires a material medium.